1. Field of the Invention
This invention relates to an improved method for injection of ions into a time of flight mass spectrometer. More specifically, apparatus which provides collisional focusing is coupled to a supersonic expansion nozzle to thereby collimate the ions in the expanding gas being emitted therefrom.
2. The State of The Art
The state of the art for injection of ions into a time of flight mass spectrometer has described the use of supersonic expansions. FIG. 1 shows where ions are entrained in a directed flow of gas in a supersonic expansion. More specifically, in a supersonic expansion, an expanding gas 12 forms a jet emanating from a supersonic expansion nozzle 10. The expanding gas 12 is flowing from a higher pressure region into a lower pressure region. Accordingly, the molecular velocities become highly organized. This is because they are directed away from the supersonic expansion nozzle 10 with nearly equal velocities, and thus fan out in a roughly cone-shaped profile 12 as if the supersonic expansion nozzle 10 formed a virtual point source for the ions.
One consequence of the dynamics of this process is that the velocity distribution of the ions (as well as that of neutral gas molecules forming the jet) is greatly narrowed compared to a typical thermal velocity distribution. This is a highly desirable condition for injection of ions 12 into an entrance 16 of a time of flight mass spectrometer 14 because it allows higher resolutions to be achieved in a properly designed instrument.
However, one disadvantage of this approach is that because the jet profile is roughly cone-shaped 12, the ion beam is not well collimated, thus negating much of the inherent advantage of the approach such as the ability to achieve high resolution.
Given that it is desirable to obtain a well collimated beam of ions, there have been various attempts to accomplish this objective. For example, FIG. 2 shows that it is possible to place an aperture 18 between the jet expansion 12 from the supersonic expansion nozzle 10 and the entrance 16 for ions to enter into the time of flight mass spectrometer 14. If the distance between the supersonic expansion nozzle 10 and the aperture 18 is large compared to the distance between the aperture 18 and the mass spectrometer entrance 16, a fairly well collimated beam can be produced. Disadvantageously, this also produces a large loss of ion intensity. Very few of the ions are directed at the entrance 16 of the mass spectrometer 14.
FIG. 3 shows that an alternative method for producing a collimated ion beam is to place an electrostatic lens 20 (or magnetic lens) between the supersonic expansion nozzle 10 and the time of flight mass spectrometer 14. A focusing condition can then be chosen so as to collimate the ion beam, thus injecting a higher proportion of the ion beam 12 into the mass spectrometer 14. Disadvantageously, the focusing condition can be chosen only to collimate ions over a limited kinetic energy range. This is undesirable because ions in the jet expansion typically have a variety of kinetic energies. This is a consequence of the fact that the ions have nearly the same velocities, but different masses. As a result, the collimation occurs only for ions of a restricted mass range, limiting the high resolution capability of the time of flight mass spectrometer to a fairly narrow range of masses.
It would be an advantage over the state of the art to provide a means whereby ions being emitted from a supersonic expansion nozzle can be collimated without having to restrict the resolution of the time of flight mass spectrometer, and without loss of ion intensity.